1. Field of the Invention
The present invention relates to a new process for making copper/tungsten and copper molybdenum composite packaging materials, and more specifically, to a chemical copper removal process that improves manufacturing and material utilization efficiency tremendously.
2. Description of the Prior Art
Copper/tungsten and copper molybdenum composites, preserving tungsten and molybdenum's characteristic of low thermal expansion and copper's high thermal conductivity, have been widely used in the packaging of microwave devices, optoelectronic components, integrated circuits, and many other electronic packaging applications. Their measures of thermal expansion coefficient, thermal conductivity, and electrical conductivity can easily be controlled by varying copper/tungsten and copper/molybdenum ratios. They can also be matched with semiconductor silicon, arsenic, gallium arsenide, aluminum oxide and beryllium oxide, etc.
Since tungsten, molybdenum and copper have considerably different melting points, it is impossible to melt W and Cu into each other at high temperatures. In other words, no eutectic will be formed. Therefore, copper/tungsten and copper/molybdenum composite material can only be fabricated through powder metallurgy. At present, there are three major techniques: infiltration, high-temperature liquid phase sintering, and activated liquid phase sintering.
Infiltration
In this technique, tungsten and molybdenum powder are mixed with a small amount of binders, pressurized and molded into a tungsten or molybdenum compact, and then sintered into a tungsten and molybdenum skeleton. Designed excess copper is thereafter attached to the surfaces of the tungsten and molybdenum skeleton and infused into the tungsten and molybdenum skeleton at temperature higher than the melting point of copper (typically 1350° C.). Precision machining will then be performed to get the desired dimensions.
An example can be found in U.S. Pat. No. 2,179,960, issued in 1939. Internationally, an example can be found in Chinese patent CN1995438, the disclosure of a process for making copper/tungsten and copper/molybdenum composites through sintering tungsten and molybdenum powder into a skeleton which is then infiltrated with molten copper.
An advantage of this technique is that during the process of infiltrating copper into the tungsten and molybdenum skeleton, most of the voids and interstices between the sintered tungsten or molybdenum particles can be filled by copper and hardly any eutectic develops. Therefore, critical measures such as hermeticity and thermal conductivity are ideal, especially the thermal conductivity, which can reach 180-200 W/mK.
The disadvantage associated with this method is that, for each and every piece of copper/tungsten and copper/molybdenum composite, thickness of the copper infiltration overflow is unpredictable and inconsistent. To facilitate precision machining, the tungsten or molybdenum skeleton needs to be at least 0.8 mm extra in thickness than the finished product. Machining allowance has to be even bigger when machining parts with mounting holes, steps, pedestals, slot and other 3D features to their desired shapes. The result is a decrease in efficiency and vast waste of tungsten or molybdenum, which sometimes accounts for 80% of the final composite and is a few times more expensive than copper. Compared to high temperature liquid phase sintering and activated liquid phase sintering, this method causes 10-45% more waste of materials. It is the most expensive technique of all considering the material and manufacturing costs, especially for irregular shaped products.
High-Temperature Liquid Phase Sintering
First, tungsten or molybdenum powders and copper powders are mixed to the designed proportion. The mixture is then pressurized and molded into compacts 0.2-0.3 mm larger in every dimension than the final product, after which it is sintered at temperature higher than 2000° C. To obtain the desired product dimensions, precise machining is performed to remove the excess copper.
An example can be found in U.S. Pat. No. 5,686,676, in which the sinterability of a copper/tungsten green compact is improved by using copper oxide, tungsten oxide or both as the copper and/or tungsten source. Sinterability is further enhanced by including steam in the sintering atmosphere.
Another example can be found in U.S. Pat. No. 6,589,310, in which the sinterability is further improved using phosphorous as sintering aid.
An advantage of this method is the efficient use of expensive tungsten, since the machining allowance will be much smaller than required by the infiltration method. It has its drawbacks, too. First, sintering temperature is high, which makes this process costly.
Sintering cost takes up the biggest proportion of total cost after material cost. Second, high temperature sintering leads to the formation of eutectic, which reduce the hermeticity and thermal conductivity of copper/tungsten and copper/molybdenum composites, the two properties that affect the reliability of integrated circuits. Thermal conductivity of final product attained by this method is only 180-190 W/mK. Finally, precise machining surface by surface and piece by piece increases complexity of manufacture and cost. It is especially the case for the fabrication of irregular shaped packaging materials.
Activated Liquid Phase Sintering:
This technique is developed to lower the temperature requirements by high-temperature liquid phase sintering. Chemical activators such as nickel, cobalt and copper oxide are included in the liquid phase sintering process. This action makes eutectic form at a lower temperature, thus reducing the temperature requirements for sintering.
Although activated liquid phase sintering lowers the sintering temperature to around 1600° C., the decrease is not significantly different from what is required by high temperature liquid phase sintering technique. Not only does it not eliminate disadvantages associated with the other manufacturing techniques, but the use of sintering activators increases the quantity of eutectics that may cause micro pores on some parts. Thus, parts produced by this technique have even lower hermeticity and thermal conductivity than those produced by high temperature liquid phase sintering. The thermal conductivity attained by this technique is only 50-170 W/mK.
Additional prior art related to this invention can be found in U.S. Pat. Nos. 343,875, 3,440,043, 3,969,754, 4,153,755, 4,158,719, 4,168,719, 4,196,442, 4,430,124, 4,451,540, 4,500,904, 4,672,417, 4,680,618, 4,736,883, 4,752,334, 4,788,627, 4,988,386, 5,009,310, 5,039,335, 5,049,184, 5,086,333, 5,099,310, 5,379,172, 5,379,191, 5,380,956, 5,386,143, 5,386,339, 5,387,815, 5,409,864, 5,413,751, 5,439,638, 6,589,310, 6,914,032, 7,063,815, 7,122,069 and 7,172,725.
All of the above techniques for manufacturing copper-tungsten composite packaging materials have their drawbacks. Extensive machining is the only way adopted by existing technologies to remove the excess copper. The amount of copper infiltration, molding shrinkage, and machinery process may cause 0.2-0.8 mm loss in size for the final product. In summary, the manufacturing efficiency has been low and the share of processing cost high, especially when fabricating products with mounting, holes, slots, pedestals and other irregular shapes.
Accordingly, there is a need for a new process for making copper/tungsten and copper molybdenum composite electronic packaging materials that reduces the complexity and cost of manufacture.